ECR (Electron Cyclotron Resonance) plasma sources are in much use for the generation of charged ion beams for accelerators and atomic physics experiments. Additionally, an important industrial application of ECR sources has developed in the field of semiconductors in which ECR sources have been used for a variety of processes required in the fabrication of integrated circuits.
A typical ECR reactor consists of a source chamber and a specimen chamber. A set of solenoidal electromagnets produces a magnetic field in the source chamber. Microwave energy and a reactant gas are introduced into the source chamber. The interaction between the microwave field and the magnetic field in the presence of the reactant gas dissociates the gas and produces a plasma. The "plasma" consists of charged ions, electrons, excited neutrals, free radicals, atoms, and other neutrals.
Typically, the magnetic field is designed to produce a magnetic mirror that confines the plasma to a particular volume within the reactor called the source region (later defined with more generality). The magnetic flux density of the field generated in the source chamber is adapted to establish electron cyclotron resonance within the chamber (i.e., 875 Gauss for 2.45 GHz excitations).
The plasma flows axially from the plasma source region into the specimen chamber along magnetic field lines where the plasma is applied to a specimen, such as a semiconductor wafer, thereby depositing, etching, or modifying the wafer with the dissociated reactants or ionized particles.
The specimen chamber includes an electrode upon which the specimen is mounted for processing. Typically, the electrode is centered perpendicular to the axis of symmetry of the magnetic field immediately below the source chamber. Hence, specimens in the specimen chamber are exposed to a line-of-sight view of the plasma source region.
ECR sources have proven useful in semiconductor processing for tasks such as plasma deposition, plasma etching, reactive ion etching, amorphizing, oxidizing and doping.
A significant advantage of ECR sources is the ability to produce a plasma discharge without cathodes so that only the material injected into the source is consumed. Therefore. ECR sources can operate continuously for long time periods and maintenance is minimal.
Despite these advantages, a number of problems still remain to be solved in the use and application of ECR sources for semiconductor processes. These problems include, inter alia, non-uniformity of application of the plasma over the specimen (wafer) surface, exposure of the wafer to damaging high-energy photon radiation and non-directional plasma fluxes resulting in pattern dependencies when etching submicron features.